Quick gear shifting is very important in automotive sports and racing because the engine cannot drive the wheels during the shift, while the clutch pedal is depressed and the engine and wheels are disengaged. The vehicle is slowing down during the entire time interval of the shift. Thus, motor racing and sports enthusiasts or professionals carefully select their manual-transmission shifters and will pay large amounts of money for a shifter which can reduce their shifting times by even a small fraction of a second.
A shifter consists basically of a lever arm pivoted near its middle for rotation about the pivot, so that the upper part of the lever arm describes part of the surface of a sphere. The upper end includes a handle, grasped by the driver to shift gears. The lower end is inside the transmission housing and includes an end, usually shaped as a ball, that engages the internal mechanisms which move the power-transmission gears about inside the housing.
The handle and the lower ball are not problem areas for fast shifting. The central pivot is the place where conventional shifters fail to deliver optimum speed.
The conventional speed shifter's central pivot is a ball joint, with a spherical or semi-spherical portion resting in a mating hollow hemi-spherical cup or socket. Thus, the lever arm can rotate about two mutually perpendicular horizontal axes. To prevent the lever arm from rotating about a vertical axis (i.e., about its own length) the conventional shifter includes short cross arms, or pins, rigidly attached to the lever arm. These outwardly-extending cross arm pins, or trunnions, slide up and down within vertical housing slots immediately adjacent to the hemi-spherical ball socket. The trunnions are co-linear (axially aligned) and in fact are conveniently made as just the outer portions of a single straight dowel pressed-fitted into a hole in the ball portion of the lever arm. The two slots on either side of the ball socket, like the two trunnions, are aligned and their sides ordinarily define two parallel planes. The slots allow the lever arm to rock side to side without rotating. The trunnions have a circular cross-section so that the lever arm can also rock to and from (in the front-rear direction) and the trunnions can rotate about their axis within the slots without binding.
A pivot such as that described above can be made low in friction by careful fabrication, lubrication, and/or material selection; but it is found in practice that the conventional ball-joint/trunnion-joint shifter tends to bind, hang up, and slow the shift, even when carefully made. It is not as smooth as it should be for really fast shifting.
Binding and hanging-up, of course, indicate mechanical interference. Since extra force is required to overcome binding, wear on the shifter is also increased.
The conventional shift lever includes a feature which has been universally adopted by prior workers in the field: namely, the various parts of the shifter pivot are geometrically aligned. In particular, the trunnions' axis passes directly through the center of the ball joint, i.e., that sphere whose surface is described by the ball of the lever arm or by the socket of the housing, when the lever arm is nested in the housing.
The present inventor has found that this prior-art alignment of the trunnion axis with the ball joint center is the cause of the binding which has plagued shifters. The alignment problem has escaped the notice of those of ordinary skill, because it is subtle and unexpected.
One reason that the prior-art shifter pivots of this type bind is best understood from FIG. 1, labeled "prior art". Two offset circles indicate the shifter lever arm ball B and the housing socket S. Two slot walls W.sub.1 and W.sub.2 confine the trunnion T. (Although cross-hatching indicates cross sectional views, FIG. 1 is schematic and is an idealized cross section.) Clearances are exaggerated for clarity. The geometrical center of the ball B is indicated by a conventional center mark C with light and dark areas within a circle about the center point. The center mark C is centered within the cross-sectional area of the trunnion T because of the geometrical alignment of the trunnion T and the center of the ball B.
FIG. 1 indicates a position of the lever arm in a gear shift at which the handle of the lever arm (not shown in FIG. 1) is an angle .theta. from the generally vertical or neutral gear position. The arm is being pulled to the right, tending to rotate the ball B in the direction of the arrow labeled A.
No mechanism can be perfectly made, and dirt and damage can interference with normal clearances. The conventional shifter, with the trunnion axis passing directly through the center of the ball, ignores this fact. Considering FIG. 1, motion in the direction of shifting (in the direction A) might cause the ball B to contact the socket S on the left side of the picture while the trunnion T contacts wall W.sub.2. Given the sure fact that the mechanism is not perfect, this could happen in many different ways depending on accident, design, and manufacture.
A line L is shown joining the two contact points. Since the trunnion T and ball B are a solid unit, and so are the socket S and wall W.sub.2, if there is any interference then continued arm motion in the direction A will exert a wedging action that tends to jam the lever arm. The jamming occurs because the shifting force A is trying to rotate the line L between two constraining walls.
The more nearly horizontal the line L in FIG. 1, the greater is the leverage of the wedging action and the greater the resistance to fast shifting. A shallow angle is not unlikely, given the close tolerances needed in the clearances of the trunnion T and ball B.
There may also be other or additional causes of binding and hanging-up in the prior-art geometrically-aligned shifters, which have not yet been analyzed here or elsewhere.
Several previous workers in the shifter field have made inventions with ball-and-socket type shifters. However, these workers have not recognized the binding problem.
Feldt et al, U.S. Pat. No. 4,569,245, also discloses a ball-and-socket type shifter but does not mention binding or shift resistance. Feldt is concerned with springs.
Warmkessel, U.S. Pat. No. 3,251,237, presents involved dimensional calculations and geometries whose object is better leverage and reduced friction.
U.S. Pat. No. 1,446,068 to Rhoads aims to reduce lubricant leakage.
U.S. Pat. No. 1,330,912 patent to Short discloses a shifter with two inverted ball-and-socket pivot bearings. One of the two bearings comprises a hemispherical cap 26 resting on a ball surface 23, and the other consists of head member 29 in contact with an inner dome surface 24 of the pedestal 21. Short also includes pins 33 whose heads slide in curved slots 34 in the cap 26.
The object of Short's invention is to provide a cheap unit using stamped parts. There is no disclosure regarding binding.
Thus, while the binding problem has been recognized, previous inventors have not even attempted to solve it.